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How do aircraft stall warning systems handle (or not) asymmetric-stall situations?
Most, if not all, new airplanes are required to have stall warning systems to (as the name should make clear) provide the pilots with a warning when the airplane is about to stall.  Most stall warning systems work not by directly detecting the rapid increase in airflow separation near the stall point, but, rather, by measuring the airplane’s angle of attack and giving an alarm if it is greater than some threshold value.  However, as the attack angle at which the wing stalls can vary by a great deal, these older stall warning systems have multiple thresholds; for instance, a particular system might sound an alarm at 15º in a clean configuration, but lower the threshold to just 5º if icing is detected (as icing promotes airflow separation, and, thus, lowers the wing’s stall angle), while waiting until 30º if the slats are extended (as leading-edge devices, such as slats, delay airflow separation, and, thus, increase the allowable attack angle).
With all this in mind, how do stall warning systems handle situations in which one wing is stalled, or close to stalling, but the other wing is not?
I can think of a number of scenarios under which this might occur:

Category 1 - The two wings have different stall angles, such as could result from...

Scenario 1: ...asymmetrical icing, or damage to the leading edge of one wing but not the other, which would increase the roughness of the damaged or more-iced wing, thereby increasing the tendency for the airflow to separate from said wing, and, therefore, cause it to stall at a lower attack angle than the undamaged or less-iced wing.
Scenario 2: ...asymmetrical leading-edge device (non)deployment, such as from failure during takeoff of the hydraulic system powering one wing’s outboard slats, resulting in the retraction of said slats, which would decrease the stall angle of the wing with non-extended or less-extended leading-edge devices.

Category 2 - The two wings are at different angles of attack relative to the airflow around them, such as could result from...

Scenario 3: ...violent, fine-grained turbulence, which could result in the airspeed of one wing being considerably lower than that of the other wing, thereby causing the horizontal component of the airflow around that wing to be considerably decreased while leaving the vertical component of said airflow unchanged, thus resulting in the lower-airspeed wing encountering the oncoming air at a much steeper angle than the higher-airspeed wing, and, as a result, possibly causing the lower-airspeed wing, but not the higher-airspeed wing, to exceed its stall angle; for instance, if an airplane is flying at an indicated airspeed 30 knots above its stall speed, and is hit by an asymmetric gust that causes the left wing to experience a 45-knot tailwind while the right wing sees a 15-knot headwind, the left wing will be 15 knots below its stall speed at the same time as the right wing is 45 knots above its stall speed, and, therefore, the left wing will stall, but not the right wing.
Scenario 4: ...a rapid yaw to one side, possibly as a result of overenthusiastic piloting or a mechanical failure, causing the airspeed of one wing to increase and that of the other to decrease, possibly causing an asymmetric stall via the same mechanism as in scenario 3 (for instance, a sudden rudder hardover would produce an extremely high yaw rate, and, thus, airspeed differential, before the pilot(s) had time to react, which could easily bring one wing, but not the other, below its stall speed).

They don't.  The Stall Protection Computer only considers overall body AOA as measured by vanes on the left and right side of the nose.  AOA is just one bit of data, and the SPC also is measuring G loads, yaw rates, speed wise acceleration or deceleration, pitch rates, etc. and also getting inputs on flap/slat position etc to allow for all those effects.  It computes a solution for presenting the barber pole on the speed tape, and the shaker and pusher (if installed) firing points based on the body AOA and all those other factors, not local wing AOA.
If you yaw a swept wing jet really hard with rudder, it will roll on its back, stalled or not, just from the asymmetric lift with the sweep (full aileron may or may not be enough to counteract it - you learn this the first time you do a V1 cut in training).
In any case you don't normally touch the rudder pedals on a jet except during takeoff and landing or if an engine quits.  Yawing the plane also makes the vanes show radically different angles because of the changes in airflow around the nose when skidding or slipping, and you will get some kind of mismatch related error message.
On a jet with heated leading edges, if you had ice on the wings, the SPC will be fooled and can't account for this, and the wing will stall at a lower AOA than the SPC's computed solution and the airplane may depart when the barber pole still shows good margin, and before the shaker goes off.  A nasty surprise.  Ice on one wing and not the other just means when it goes, before shaker, it will roll really hard.

This may be right on some aircraft but most light aircraft there is no stall protection computer, rather much simpler systems.

Obviously yes, but I'm pretty sure Sean's talking about airliners with things like mention of slats etc.

You may want to qualify your answer then @JohnK.

@JohnK: Yes, that's what I was thinking of.

The pilot handles asymmetric stalls, this is what a pilot is there for. Along with all of the other corner cases that glorified calculators just can't be relied on to handle. A pilot with adequate manual flying practice can easily detect abnormalities in the response of control inputs and (in the USA)they are actively trained for these different configurations and conditions. I can't count the number of stalls and spins I have purposefully initiated during my training, in many different configurations. (flaps no flaps, level, in turns, full thrust, idle thrust, different altitudes) The typical student private pilot will have stalled in at least two configurations by the the third hour of training as part of the curriculum.
A spin is just an asymmetric stall allowed time to fully develop. Suspected icing calls for different configuration and speed range, generally includes reduced angle of attack. The selection of the icing AOA is a pilot decision, in small aircraft this is done simply with different flap settings and minimum speeds, in advanced aircraft the pilot must set the stall calculator its icing mode. In any case pilots have been handling this for 100 years.


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Why don’t aircraft undergo contaminated-runway landing-distance and rejected-takeoff certification testing?
All aircraft have to undergo testing to determine how much runway they need in order to safely land and come to a stop, or to safely reject a takeoff.
They do not, however, have to undergo this testing on anything other than a perfectly clean, dry runway.  Instead, to get landing/RTO distances for contaminated runways, they multiply the dry-runway distance by some arbitrarily-determined factor (precisely which factor depends on what the runway is contaminated with - water, slush, ice, wet snow, dry snow, sand, Teflon, Jet A-1, ping-pong balls, whatevs), and then everyone hopes that this actually gives them enough of a margin for landing on contaminated runways.
Why not simply run landing-distance and rejected-takeoff certification tests on contaminated runways, as well as on dry runways, and directly measure how far it takes to stop on a wet or icy or whatever runway?

I can see a problem with the "scientific" nature of that type of testing. For example they would have to ice a runway (a big one), and those numbers would only be meaningful in the exact same conditions. What if the ice is really cold or warm? What if there is a light dusting of snow over it? Or snow, how thick? 1", 6"? Heavy snow? Dry light snow? I think this would add years and millions to testing that really wouldn't have any way to say "yeah, this is situation CR15, we are going to need X feet to stop". If you are concerned with stopping, don't land there.

Related, a proposal from the Civil Aviation Administration of China for Certification of Aeroplane Takeoff and Landing Performance on Contaminated Runways

Baseline performance data was developed some years ago by an FAA test program called the Joint Winter Runway Friction Program to develop mathematical models of the effects of the different types of surface contamination.  Manufacturers apply the models to develop their own aircraft's performance correction factors for various contaminated runway conditions.
Otherwise, as Ron comments, the manufacturers would have a massive test burden to develop customized data for a huge range of contaminants, and would have to go find the test conditions somewhere for each, adding eleventy jillion dollars to the typical development program.  The mathematical models, along with the safety factors built into the calculations, do the job just as well.
This Advisory Circular talks about it.

Can you provide a citation for your figure of eleventh jillion? :P

The proper technical term is eleventY... eleventY... ;D

Damn autocorrect! That’s what I typed. I guess my iPad doesn’t have “eleventy” in its dictionary :P

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Why can't wing-mounted spoilers be used to steepen approaches?
In a comment on this answer to my previous question about tail-mounted airbrakes (lack of), @JohnK states:

You won't see flight spoilers (speed brakes) being used to steepen approaches. You have to be 10kt above Vref to extend them on a CRJ900 and you shouldn't be that hot in the first place, and you aren't allowed to use them at all below 300 ft.

Why do you have to be going abnormally fast to slow down with the spoilers, and why can't they be used at low altitude (especially given that, just like with airbrakes, it would seem to me like spoiler extension would be a safer method of slowing down than pulling the throttle all the way back would be, since, should a go-around become necessary, the spoilers can retract a lot more quickly than the engines can spool up from idle to TOGA)?

Engines are never at idle on approach until the landing flare for that exact reason.

They ARE used to steepen approaches - just ask anyone who's ever flown sailplanes.  For why particular planes don't use them, or allow use only in particular circumstances, you'd have to ask the designers.  But I'd suggest that a great part of the reason they aren't used in commercial airliners is that throttling the engine back produces a steep enough approach - it's the difference between the 12:1 glide ratio of a typical airliner versus the 50:1 or more of sailplanes.

Spoilers are called spoilers, because they work by spoiling lift.
To slow the aircraft, or make it descend faster, it needs to dissipate the energy and that means increase drag. Disrupting the airflow over the wing creates a lot of drag due to stagnation behind the spoilers, and additional induced drag as the lift distribution is pushed further from optimal. After landing disrupting lift also puts more weight on the wheels, which makes the brakes more effective.
The price for it is, however, is that the aircraft flies at higher angle of attack to compensate for the lift reduction, and that means closer to stall. That means the aircraft must not fly as slow to maintain the safety margin, and shouldn't use spoilers at low altitude where there would not be enough height to recover in case of stall.
Gliders to normally use spoilers all the way to landing. Without engine, it is their only way to regulate the rate of losing energy. However a light slow glider can afford flying close to stall, because they can be recovered very quickly after retracting the spoilers. Their spoilers also tend to be relatively small, so they don't allow that steep approaches.
Airliners are another matter though. They have a lot of inertia, which makes them respond to control input rather slowly. Combined with their much higher speed it makes stall due to spoilers left out a bit too long a much bigger problem. Their spoilers are also larger as their primary functions are dumping lift on the ground and emergency descent, which is normally flown at maximum allowable speed.
As mentioned by @user71659, there are some aircraft that do use spoilers for steep approach, e.g. A318. But A318 has unusually low stall speed as it uses the same wings as A320 that are large for its weight, and its flight control computer is programmed to retract the spoilers (and spool up the engines) if it gets too close to stall.
It would be also possible to have the spoiler extend only a little bit, or only some of them, for steeper approaches. However, steep approaches are rare, so they are not worth the additional complexity in the control system for the purpose for most aircraft manufacturers and airlines.

I don't really follow the logic here. Admittedly my expertise is on gliders, but here it is: if you come with spoilers open yes, the stall speed is higher, but you can also fly closer to stall speed because closing the spoilers is an instantaneous way to recover lift. In gliders this works extremely well, shorter and smoother landings are achieved by coming with spoilers open on final.

@Jan Hudec, no insult intended but your first paragraph reads like a case study in missing the point.  You are saying that because lift is reduced the aircraft has to fly at a higher angle of attack and increase speed to counteract.  Technically you are correct, but the whole point of using spoilers or speed brakes is to slow down and increase the rate of descent.  Why counteract the very effects you are trying to achieve?!

@MichaelHall: I'll try to reword it. The point of using spoilers is indeed to slow down and increase the rate of descent, but that means dissipating energy by increasing drag and this is a way to create a lot of drag. But it has that downside. I didn't really want to explain the purpose, only the side-effect, because that is relevant for the question at hand.

@MartinArgerami, I am basically taking the quoted limitation of CRJ900 at face value. If spoilers are not allowed below Vref+10, there are not many possible reasons besides that it is considered too dangerous.

@JanHudec, drag isn't really a side effect though, drag is the primary effect.  Slowing down is actually a side effect of the drag, but again if you put them out then that's what you want to do obviously.  Your edit is better, but I still don't like this sentence:  "higher angle of attack means closer to stall. That means the aircraft has to fly faster to maintain the safety margin"  Again, technically there is nothing incorrect with this statement, but when would you ever extend spoilers and then accelerate to increase stall margin?

@MichaelHall, I didn't say drag is side-effect. I said the stall speed increase is side-effect, but it is that side-effect that is important for this question.

@JanHudec, OK, the distinction between side effect and primary effect isn't really key to my point. The line I quoted makes it sound as if a pilot might deploy spoilers to slow down, then re-accelerate to increase stall margin because AOA had increased to maintain altitude.  This is not a realistic scenario as it makes no sense. If you pop the boards it is either to slow down, or come down faster in which case you'd dump the nose to hold airspeed or maybe even accelerate. However, the acceleration wouldn't be to increase stall margin because you wouldn't have increased AOA. –

Induced drag does not increase due to a higher angle of attack, but to the change in lift distribution over span. I am sure you meant to say this, but the first paragraph is awfully easy to misunderstand.

@PeterKämpf, to be honest I am not too sure what happens. However, I think even if spoilers were extended a bit over all span (and airliners have them on most of it), it would still increase induced drag, because the disrupted flow is not being deflected down, so the remaining flow must be deflected more, taking away more energy. So in a sense it is change in lift distribution between upper and lower surface rather than along span.

Spoilers retard airflow and, consequently, circulation. The speed brakes on gliders effectively cut a single wing in three small, parallel pieces which all have their own induced drag. Local induced drag is circulation strength times circulation gradient over span. There is no need to look at the upper and lower surfaces in detail, only the sum of it all counts. Airliner spoilers cause less added induced drag but lots of pressure drag from separated flow, especially over the backward-facing upper flap surfaces. Induced drag is a smaller factor.

They can. The Airbus A318 and Embraer E190 both do this in steep approach mode. You can see spoilers extended in this video of a A318 landing and a similar approach in an E190 at London City Airport.
Going back further, the Lockheed L-1011 did this with a system called Direct Lift Control. When landing flaps were selected, spoilers raised a set amount. Small movements on the yoke raised or lowered the spoilers instead of adjusting the elevators. This made for a very smooth approach and landing.
The common point is that these aircraft use electronics for spoiler control. It's too complicated to do this in a cable flight control system. You need to have the system retract the spoilers quickly in cases like go-around and stalls. As I recall, steep approach mode on the A318 activates a different set of gains on the flight control loops for better handling qualities in that configuration.
Finally, a number of fighters have the same system, and some glider pilots do this manually.

The F14 had DLC to help with carrier approaches. The A6 used spoilers instead of ailerons. Some pilots would finesse them and waggle the wings to get the left and right spoilers alternating, to drop the aircraft down onto a glideslope without messing with the pitch angle.

Ditto the EA-6B!

The main issue is the need to manage the vertical sink rate of your 100000+ lb aluminum trash can and the risks of hard landings.
Although they are often called speed brakes, the main effect of flight spoilers is to increase sink rate (without speeding up).  On a landing approach at typical jet speeds of around 130kt you don't want to be descending any more than about 700-800 fpm (about what you get at that speed on a 3 degree slope). More than 1000 fpm is typically considered an unstablilized approach and necessitates a go around if at that sink rate below 1000 ft with many airlines.
In the landing configuration on a 3 degree ILS glide slope you are flying 99.9% of the time, you will normally need considerable thrust on, and small pitch and thrust adjustments are more than sufficient to precisely maintain the slope and sink rate.
The problem is having high sink rates close to the ground and having to arrest the sink at the bottom with all that mass/intertia doing downhill, when the only tool you have to do so is to increase overall lift by pitching up.  Once the high sink is established, retracting the flight spoilers only helps a little and you have to pitch up (and add thrust, but that it is going to be delayed).
On airliners with leading edge slats and multi-slotted flaps that are super draggy in the landing configuration, unless you have a significant energy margin above Vref, the ability to arrest the sink rate with pitch is limited and there is little margin for error.  Get a little bit below Vref and pull pitch, and not much happens until the thrust comes up.  As a result there is a high risk that you'll continue sinking right through the landing flare and hit hard.

I've flown both the original CRJ200 with hard leading edges and the later 700/900 versions with slats, and the 200 is actually the more forgiving airplane to land because it is less draggy in the landing config and likes to coast along instead of dropping out.  You go to idle at 50 ft on a 200 but have to leave power on down to the start of the flare in the slatted ones.  Between that and the trailing link gear, nice landings with the 200 are effortless.  Hard landings happen to pilots transitioning from the 200 to the 7/900 from pulling power too soon.

The answer is yes, you CAN use spoilers (in a flight position!) to steepen an approach.  The aircraft remains fully controllable and can be flared prior to touchdown with the spoilers in this position.
While it is safe, legal, and reasonable to do this, it’s considered a faux pas among professional airline and corporate pilots as high performance flying interferes with passenger comfort, especially for nervous flyers.  Making the trip as uneventful and smooth as possible is part of a pilot’s job as well.

What it boils down to is brakes (aerodynamic, wheels, and reverse thrusters) and weight.
First the strategy for the massive airliner:  come in as flat and slow as possible, set it down, and bring 200 tons to a stop.  There for the best application of airbrakes is to have them as lifting devices.  Slats and flats create an under and over cambered wing, which, while draggy
(good here), produces the most lift at slow speeds.  Steep approaches are out as the physics
of inertia make this a "case study" of the dangers of rounding out at low altitude and crushing the landing gear with too high a rate of descent.  Set it down easy, then brakes, reverse thrust, every airbrake available (including spoilers). A plane landing at 150 knots requires more than 2 times the braking force as a plane landing at 100 knots.
With a spoiler, you have to go faster to get the same amount of lift.  With a flap, slower.
Same drag.  So in this case, you pivot the airbrake down.
Now for the sailplane strategy:  steepen the angle of descent.  If it is 50 feet high at the runway threshold it may sail right over a 3000 foot runway!  We want drag and less lift,
but how do we avoid stalling? By speeding up.  Here, a little extra speed is no problem.
Now you get the same lift at higher speed if you need it, rather than pitching up.  Going faster in any case means less efficient gliding and more efficient air braking. The extra speed will not be a significant factor once the plane is down (getting it to stop).
So in this case, you pivot the airbrake up.
Same airbrake!  Heavy, slow, flat - down.  Light, fast, steep - up.
Depends on what you fly.

what do you fly?  Do you fly gliders?  I have flown gliders only a few times but I find your paragraph above very confusing...

I don't like the paragraph regarding an airliner either.  Nobody flies as "flat and slow as possible".  Where did you come up with this?

@Michael Hall multiengined large aircraft come in (see John K) at 3 degrees flaps down power up to minimize their rate of sink.  I never did this with a single engine because you are below glide slope if the engine quits.  But rate of descent with a very heavy aircraft is more easily controlled with a shallow, power on glide slope.  Indeed, even with a 172, it makes the round out much easier.  Contrasting with the sailplane, who makes its wing "smaller" by spoiling lift, so it can steepen its angle to make the runway.

The part about gliders also sounds iffy to me. Gliders approach with spoilers deployed to offset their lack of go-around capability. If, due to any reason, the glider finds itself landing short of the runway, it can retract spoilers and extend the approach. I believe you have the right idea, but the reasoning about the cause could be cleared up a bit.

Well, that's the flip side of going too long.  Again, the spoiler makes the wing "smaller", giving the plane a higher stall speed and more drag.  It basicly works the fast side of  Vbg.  So, if you're going short, retracting spoiler will decrease the glide slope and increase the glide distance more lift and lower stall speed.  So: spoiler - clean - flaps : decreasing stall speed. Spoiler - flaps (equal sized) - clean increasing range. Flaps and power (slow side of Vbg)  lowest and slowest glide slope.

"retracting spoiler will decrease the glide slope and increase the glide distance"  This makes perfect sense.  I can't say the same for your last summary sentence...

@Michael Hall  My fantasy aircraft carrier landing was to come in a little high and chop the throttle to catch the wire.  But, could I leave the engine spooked and pop the spoiler instead?

You can do whatever you want in a simulator or fantasy world, but that isn't how it's done...

OK, just trying to find an application for the question. Spooled, damn spell check.  Simulator might be better.

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Could the Olympus 593 have been windmill-restarted during supersonic flight?
The Rolls-Royce/SNECMA Olympus 593 (which powered the Concorde) used a complex intake-ramp system to improve the engine's efficiency and prevent an engine shutdown or failure during supersonic flight from tearing the aircraft apart; it did the latter by dumping almost all of the intake air overboard via a set of spill doors, allowing it to bypass the inoperative engine and thereby avoid the considerable drag generated therein.
On the other hand, a windmilling engine restart (one of only two midair relight methods possible for the Concorde, as it had no APU - the other being the use of bleed air from a still-operating engine - and the only one available in the event of a quadruple engine failure)1 requires that ambient air be forced through the engine in order to get it spinning, which would seem to necessitate closing the spill doors and not dumping the intake air overboard.
Could the Concorde's engines have been windmill-lit during supersonic flight, or would the aircraft have had to have been slowed to subsonic speeds prior to attempting a windmill restart?
1: While admittedly an unlikely scenario, there are multiple possible ways for this to happen other than fuel exhaustion; possible examples would be if a Concorde inadvertently flew through a volcanic ash cloud, or suffered fuel contamination, or experienced an upset at high altitude.

I couldn't find a reference for this so it's largely speculation, but Concorde may not have been able to maintain supersonic speed on three engines, so your question would be academic.


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Why did Lockheed abandon the Constellation II?
It’s well known that Lockheed never designed a first-generation jetliner.
Not as well known: they actually did!
The L-193 Constellation II, a quadjet with four tail-mounted engines (a configuration Lockheed would later reuse for their Jetstar business jet, and which was otherwise seen only on the VC10 and Il-62), was designed in both an airliner and a tanker version, and it actually won the U.S. Air Force’s 1955 jet tanker competition, beating out Boeing’s KC-135.  However, since the Constellation II was still a couple years away from flying, the Air Force bought a few KC-135s for use in the meantime, soon decided that they were good enough for the purpose, and eventually cancelled the Lockheed order.
At which point Lockheed abandoned the Constellation II entirely.
Which is odd, because Douglas, in pretty much the same situation at about the same time, didn’t abandon the DC-8 when the prospect for a tanker version thereof vanished in the KC-135’s jet blast; they merely redoubled their efforts with the DC-8 airliner, and went on to great success in the marketplace.  Lockheed, on the other hand, gave up on their jetliner when they lost the Air Force tanker contract.  Why?

Douglas probably already had a few commercial operators interested and close to signing orders when their tanker bid fell through.
Lockheed didn't, and decided to not dump more money into what could very well become a black hole project that'd never pay off.
And it may well be that they decided the design wasn't feasible for commercial operators. Looking at the picture on Wikipedia, almost the entire rear third of the fuselage would have no place to fit doors or windows, making the aircraft useless for passenger operations.
And the air cargo market at the time was a lot smaller than it is today and still saturated with former military aircraft retired after the Korean war and WW2.


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How do balloon pilots avoid birdstrikes?
Balloons, by their very nature, are at the mercy of the wind, and can’t maneuver out of the way of other objects except by (very slowly) climbing or descending.  For this reason, mid-air collisions are fairly common whenever and wherever large numbers of balloons occupy small volumes of airspace (especially because this inability to take evasive action essentially restricts most balloons to uncontrolled airspace, which, in most places, is a very small layer immediately above the ground); fortunately, as balloons are very slow1 and very soft, balloon-balloon MACs are, generally, completely harmless (except in the extremely rare cases where a balloon rises into the basket of a balloon above it and gets its envelope ripped open).
Birds, on the other hand, are not necessarily at the mercy of the wind, are largely concentrated at the low altitudes where most balloons operate, are far more numerous than balloons (which is probably still the case even in the middle of a balloon gathering), and tend to come with a lot of inconsiderate pointy bits (beak, claws, probably other things too).  Unlike with heavier-than-air aircraft, where a birdstrike usually just means a few dents, or, at worst, some unscheduled engine maintenance (although, as always, there are exceptions), a balloon could, if hit by a bird, easily suffer a catastrophic envelope rupture, followed quickly by (at best) a hard forced landing, or (at worst) rapid unscheduled lithobraking.2
How do balloons and birds avoid each other?

1: Not necessarily slow relative to the ground, but definitely slow relative to each other (since they’re being pushed by the same, or mostly the same, wind), which is what counts in a MAC.
2: Like the chapter in The Twenty-One Balloons where the protagonist’s houseballoon is punctured by a seagull, and he has to make a forced landing on a beach.

What is the maximum horizontal speed of a balloon? The maximum (horizontal) relative speed is very low when compared to airplanes, I assume.

Hot air balloons have a huge amount of air in them. Would a bird-sized hole in one really be such a catastrophe?

Planes go very fast. A bird could easily not see one coming or would be unable to react in time. A large, slow, round balloon on the other hand? A bird would have to be an idiot to fly into it.

Why would a bird fly into a balloon?

I've got over 100 hours in balloons, crewed for around 200 more flights and used to work at a hot air balloon MRO. I have never noticed or heard of a bird strike or seen any damage from one. I have had multiple mid-air collisions with other balloons. Have seen a balloon with bullet holes (from someone who thought it would pop) and many other tears and rips.
Hot air balloons are made of ripstop nylon and are not pressurized, they do not pop like latex balloons and can handle some pretty extensive damage and still fly. The ripstop fabric has strong reinforcement threads woven into it in a diamond pattern, this helps prevents rips/tear from propagating once the tear hits a reinforcement thread.
Balloons are made from many panels of ripstop (see figure below). These panels are sewn together using a French Fell seam (the same stitch that is used on the inside seam of jeans). If a rip does run the full length of the panel, it will almost always be stopped by the seam and will not transition to the next panel, just like cracks in riveted aircraft skin rarely jump to the next panel.

(Source)
Running on top of every vertical seam, and many of the horizontal seams is a thick nylon webbing called Load Tape. This load tape is what actually carries the majority of the structural loads of the balloon. Due to the strength of the load tape, it is very hard to tear, and provides strain control for the envelope, which would further prevent any rip from running past a seam with load tape.
A balloon will also not deflate or lose altitude from a hole any bird could put into it. When the burner runs, not only do you add the burner exhaust into envelope, but you also heat the air around the flame and cause it to flow into the envelope. A good way of visualizing this is to watch a balloon "standing up," although a fan is used to put air in the envelope, it is often less than half full when the pilot starts the burner to stand it up, the rest of the air is provided by the burners exhaust and the flow of hot air into the envelope. To put in to perspective how big a hole would have to be to affect a balloon, the parachute vent valve at the top of an average size (90,000 cubic feet) balloon is about 35 feet in diameter and can be pulled down several feet in flight in order to descend (and it doesn't cause you to fall out of the sky).
Of course there are limits to how large holes and tears can be before you take off, but there isn't much risk of a bird tearing a large enough hole to prevent continued safe flight and landing. And of course, a bird is more likely to land on the balloon than fly into one.

The airspeed of a hot air balloon is always... zero.  So it's completely up to the bird to do the avoiding.  Which shouldn't be too difficult for the bird, on account of this humongous giant brightly coloured monster thing floating in its way.  Ya gotta be a pretty dumb or drunk bird to run into one.

Nitpick: the airspeed is only zero in equilibrium. In reality, the wind gusts and changes direction. But, yeah, close to zero and what you said.

Drunk?  [googles...] Huh. Avian intoxication is well documented.  https://www.audubon.org/news/what-happens-when-birds-get-drunk

First off balloons dont very gently ascend or descend, as a fixed wing pilot you need to understand never fly directly above a balloon, our rate of ascent is multiple times yours 1000fpm not a problem, easy and is the normal reaction from a pilot in trouble above a certain altitude (few dozen/hundred feet), so dont try to out climb us, we will all be in trouble.  go around.
we go the speed of the wind, while birds do fly into my picture window of my house, because it sometimes looks like it isnt there and they want to land in my living room.  they dont often fly into brick buildings just for fun nor will they fly into a massive balloon that is going really slow compared to them.
you dont launch unless the winds are favorable, ideally under 10 mph on the ground and that speed or less during the flight as you often fly in the morning and upper winds drop so while you can get up to 20+knots that is still slow compared to a bird.
If the bird cant see the 2/3 story building in front of them even the largest bird is not going to cause a serious problem.  the are likely to bounce off the fabric not go through it, if they go through the hole wont be that big, as answered it is not pressurized like a helium latex balloon, the whole thing is leaking air (very tiny amounts but non-zero) unless brand new and even then...its not like a tv show I saw where a small 1 inch hole near the throat needed macgyver to climb up and fix it.  that is just silly.  by the through you are legal to fly with small holes at above the equator the rules change but it is per manufacturer.  you can technically fly with a big rip in the side, being rip stop nylon it doesnt increase unless the fabric is in bad shape which means you didnt pass inspection which means you are not flying so that is not a problem.
I had heard of an RC plane intentionally fly into a balloon because they were tired of balloons flying over their field on the weekends, not realizing this is a federal offense, the biggest problem for the pilot was a plane falling down through the throat with the prop spinning and not getting injured from that.  Same would be for a bird if it were even remotely possible to break through, I cant think of a bird that is that fast, narrow with that sharp of a pointed beak that could even attempt such a thing and would have to do it intentionally.  I dont think a hummingbird has the mass to get through.  anyway if that were to happen then the next concern is the bird trying to get out through the throat.  the hole in the balloon would not be too much of a concern, could still land the balloon, etc just couldnt fly again until repaired.
never heard of a bird strike, would have to be a pretty dumb bird, and likely would just bounce off, if it were to penetrate likely would be only the beak to the face, then the bird would wiggle out, that hole would not be a concern other than found during pre-flight and not get to fly or found during annual inspection and require a repair.
EDIT
re-reading your question it is very uncommon for mid air collisions even during events like fiesta, but that depends on your definition, because of how close quarters have gotten, four balloons per square at fiesta or more, you rub against others all the time and rub on the way out, sometimes basket to envelope, but usually that is a pilot mistake.  You are most likely to see a balloon to balloon collision at fiesta in the air specifically but not at other rallies as others only have one wave/row of launches not three like fiesta.  Once aloft mid air is very very very rare even at fiesta as you cant steer your way into one.
Your assumptions in general about balloons seem to be off.  Rate of climb, number of mid air collisions at rallies, etc...
Balloons are both quiet and noisy, the silence to loud would keep birds away if any were nearby.  The inflation fan is noisy and would keep them at a distance, the inflation is noisy would keep that at a distance, and it is a flying house that is usually bigger than the trees that they can see and not hit but instead land on.  It is quite hot as you get close they could no doubt feel that, etc.
Birds are not a concern.
Other balloons are not a concern.  Any more than other planes are an airport to a fixed wing pilot.  Everyone follows the rules/procedures there are no issues.

Re "while birds do fly into my picture window of my house, because it sometimes looks like it isnt there and they want to land in my living room. they dont often fly into brick buildings just for fun"  -- +1 on that!


text:
Why were the space shuttle’s flight controls only certified for low airspeeds?
Somewhat ironically for a vehicle designed to reach orbital speed, the space shuttle’s flight control system was only certified for airspeeds up to 333 KIAS, and exceeding 470 KIAS (the maximum allowable in dire emergency) would have led to an immediate loss of control.  This constraint was the reason for at least two, and possibly three, of the low-survivability zones in the event of a triple engine failure (go to page 14 of the linked PDF and look at the number of times it says “excessive EAS during pullout”1).
In contrast, essentially all modern jets have flight controls certified to well over 500 KIAS, and capable of safely controlling the aircraft at airspeeds considerably higher still.
Given the lethal effects of an overspeed-induced loss of control during an emergency situation, why were the shuttle’s flight controls certified with such a ridiculously low maximum airspeed?

1: Technically, it uses KEAS (knots equivalent airspeed) instead of KIAS (knots indicated airspeed), but, assuming that your airspeed indicator is properly calibrated and the pitot-static system on your aircraft is working properly, KEAS=KIAS.

Where do you get 500kt?  Vmo on most jets is in the low 300s.  Speed limits are generally related to Mach related issues or flutter margins.

This seems a bit like asking "why wouldn't the brakes on my truck work at F1 car speeds?" You're comparing two very different aircraft designed for completely different purposes and flight parameters The comparison may simply not make sense.

@JohnK: The CRJ700 has a maximum cruise speed of 470 KIAS, and I would imagine other jetliners would be similar.

@Sean You have a basic misunderstanding.  That's true airspeed not KIAS.  What matters is indicated/calibrated airspeed for certification limits, what shows on the speed tape or indicator in the cockpit.  The RJ's Vmo is 315, 330 and 335 KIAS at various altitudes, or at higher altitude, an Mmo of Mach .8 up above FL250 and .85 above FL320, (where Mach # is limiting).  The Shuttle's Vmo is actually comparable to other jets.  If the Shuttle is indicating 330 kt at say 80000 ft, the TAS is gonna be over 700 or 800 kts or something like that.

@Pondlife: Your truck has no prospect of ever reaching Formula 1 speeds, whereas the  space shuttle could easily have exceeded 470 KIAS if something went wrong.

From what I understand the limits seem to be related to contingency maneuvers such as RTLS or TAL where the external tank was still attached. These were only possible after the solid rocket boosters had been jettisoned, but while the external tank was still attached.
I couldn't find any altitude references for RTLS, but according to this
TAL required the shuttle to fly to around 350.000 feet. RTLS would have been at a lower altitude and required a 180 degree turnaround. Doing these maneuvers with the ET places a higher stress on the vehicle resulting in black zones. Wayne Hale also mentions this in an old blog post, but doesn't go into specifics.


text:
Do aircraft performance figures include corrections for areas with especially strong or weak gravity?
The strength of gravity at the Earth's surface is not constant, but varies from place to place by as much as a few percent:

(Image by NASA.)
As a result, an identically-loaded aircraft weighs less in (for instance) southern India, Uyghurstan, the eastern Caribbean, or the Hudson Bay area than it does in (for instance) Indonesia, Greenland, Liberia, or the Russian Far East, which should (in theory, at least) affect its operations in several different ways:1

An aircraft in a lower-gravity region won't need to generate as much lift in order to become, or remain, airborne, allowing it to take off and land at slightly lower speeds, to climb slightly more steeply, and to cruise at slightly higher altitudes.
An aircraft in a lower-gravity region can be loaded slightly fuller without exceeding its maximum allowable takeoff weight (potentially giving it a range advantage over an aircraft launched from a higher-gravity region, if the extra capacity is used to carry more fuel), and can land slightly fuller without exceeding its maximum landing weight and risking structural damage.
An aircraft landing in a lower-gravity region will have less weight on its wheels, slightly reducing the maximum braking force available from the wheelbrakes (which might well cancel out the landing-weight advantage mentioned in the previous bulletpoint), and, thus, requiring a slightly longer runway.

Do aircraft performance figures and calculations include corrections for local and/or regional variations in the strength of gravity, or are these variations simply considered too small to bother accounting with?

1: This is by no means meant to be an exhaustive list.

1 gal is about 1/1000 of  1 g. 1 milligal is 10^-6 g.  100 milligal = 10^-4 g. I do not see several percent of variation.

The variation of gravity is very small. For an A380 with MTOW of 590t, this makes just 0,6t. More noticeable is the lifting effect of earths rotation, which makes everything about 0.34% lighter at the equator that at the poles. That makes about 2t for the A380. Still not much.

@sweber: I agree, not much, but, given that airliners frequently operate within a few hundred kilos of their maximum allowable weights (otherwise, they're not making as much money as they could be), that could still be the difference between (say) being able to load one more cargo container into the aircraft, or having to leave it by the side of the runway.

No they don’t. Any variations in gravity are swamped by variations in temperature, pressure, density, winds, runway surface, runway slope, runway conditions (wet?) etc. that all have a major impact on performance.
But it is an interesting question, and I learned something by following the link. Thanks!
I know it does have an impact on navigation for submarines, which use variations in gravitational fields in their inertial nav algorithms.

Some fuel performance calculations I know take latitude/longitude into consideration; however that only considers gravitational variations on an ideal ellipsoid (probably WGS-84), not the detailed mapping in original question.


text:
Why do cargo aircraft still have floors?
Even with dedicated cargo aircraft built as cargo aircraft (rather than converted from a passenger-carrying configuration), the cabin is virtually always still divided into an upper and a lower lobe (which would, on a passenger aircraft, be the passenger cabin and the cargo hold, respectively) by a floor located a third to a half of the way from the bottom of the fuselage, drastically reducing the maximum size of the cargo that can be carried and making it much harder to transport round outsize cargo (such as, for instance, large high-bypass turbofan engines as are used on essentially all modern jetliners) by air.  Given that the essential structural members of a large modern jet are

the skin itself;
longitudinal longerons running the length of the fuselage just beneath the skin; and
circumferential frames encircling the fuselage just beneath the skin;

it should be possible to remove the floor, and thus dramatically increase the freighter's ability to carry very large objects, without significantly compromising the fuselage's structural integrity (the middle of the cabin would still be partially occluded at the bottom - or the top, or the middle, depending - by the center portion of the wingbox, but there would, nevertheless, be a considerable increase in the outsize-cargo capacity ahead of and behind the wings).  Why isn't this done?  Is the floor left in to facilitate possible future conversion to a passenger aircraft?

There are quite a few that have a very "low" floor, remember there is still stuff running through there, hydraulic systems, gear bays, center fuel tanks, fuel transfer lines/pumps, avionics bays, APU compartments, etc. that need to be in the main fuselage as well

How do you load general cargo on a round floor?  Even the Beluga pictured below, though it's carrying round fuselage sections, has a flat floor for the cradle's wheels to roll on.

Because most cargo aircraft are derived from passenger aircraft. All dedicated cargo designs have no extra floor: They carry the payload on the fuselage bottom structure.

Comparison of transport aircraft cross sections aft of wing drawn to the same scale (picture source). The C-5 and C-17 are dedicated military cargo designs and their round bottom is not only for aerodynamics, but also to fair the structure needed for carrying the cargo floor bending moments. The Boeing 747-400 is a passenger aircraft which is also used as a cargo aircraft.
Even many aircraft which seem to be specially designed for cargo duties have in fact a passenger-carrying origin, and applying more than structurally superficial changes would cost too much for the limited number of aircraft needed. Most freighters started their life as passenger aircraft and only small changes are efficient; redesigning the fuselage is completely uneconomical.

There is also an associated down-side to having a floor as it needs to be structurally reinforced to carry the weight of the cargo (as opposed to the weight of passengers). That is supposedly one of the reasons that an A380 Freighter is about $1B away from being a reality as discussed here and here

@nodapic: After that Turkish DC-10 accident, the floor has to carry the full pressure difference in case the cargo area loses pressure. This is easily enough to support the weight of the cargo, too.

interesting connection. Does that only apply to older aircraft where that was not part of the original design and you thus have to reinforce the floor during the conversion? Conversion houses talk about structural modifications to the floor being part of most STC, see here and here

Because the overwhelming amount of cargo is fairly small and floors allow easy loading of both decks at the same time which is more important than the ability to load big cargo on occasion. By having floors you can load standard cargo containers quickly and efficiently. It also gives you the ability to standardize the containers with a reasonable degree of reuse most importantly they can fit into the lower deck of a passenger aircraft with out modification. Passenger airlines make a lot of money moving commercial cargo.
When it comes to flying around big, cumbersome cargo there are simply aircraft designed and built for such a task like the Airbus Beluga

(source)

+1 for the beautiful pic! :)

On most transport aircraft the floor structure, especially the beams along the sides, is a key part of the fuselage as a load bearing pipe whos lower half is under compression and benefits from the stiffening.  Also the space under the floors is full of components, avionics, ducting yadda yadda.  Ya gotta put it somewhere.  Lots of airliners have underfloor cargo holds though.
The passenger aspect is also a factor.  You could design a cargo only airplane with only a very skinny floor section as low as possible that would take specialized circular containers, but then that's all it could do.

Employees walking through aren't always the most cautious. From experience working at a cargo airline, it is very easy to misstep and fall through and land on something expensive when the floors are up for maintenance.

I think the question is removing the floor totally, leaving a big round space for cargo.

@Anilv: Your intuition is correct.


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Why is the Departure leg of the traffic pattern being mistaught as Upwind?
I hear a LOT of newer pilots and CFI's erroneously referring to the Departure leg of the traffic pattern as the Upwind. Ref. A.I.M. Sec 4-3-2.  Actually had a Tower controller tell me "I'll call your turn from Upwind"  I was clearly on Departure...just left the runway on runway heading. I called back "1234x is on Departure, sir" No reaction. Thank you.

Someone posted a letter from the FAA responding to inconsistencies in documentation on this topic.  http://sportysnetwork.com/airfacts/files/2017/03/Signed-3-20-17-Response-to-Shyam-Jha.pdf

Worth noting some countries outside the US (eg Australia) have the upwind leg only and don't define a departure leg.

In a closed traffic pattern I have never heard the term “departure” used in place of upwind.  Departure is appropriate if you are leaving the vicinity of the airfield, otherwise it is out of context relative to a box pattern.

@BowlOfRed https://web.archive.org/web/20180406055605/http://sportysnetwork.com/airfacts/files/2017/03/Signed-3-20-17-Response-to-Shyam-Jha.pdf

From an ATC perspective: the 7110.65 Chapter 3 Section 8 "Spacing and Sequencing" does not mention any "upwind." Instead it includes such phraseology as "MAKE SHORT APPROACH," "GO AROUND," or "EXTEND DOWNWIND."
As @757toga points out, the P/CG definition of "traffic pattern" does not mention a "departure" leg at all. It does define the "upwind" leg as being "parallel to the landing runway in the direction of landing." This begs the question: Is a line parallel to itself? If a line (or leg) can be parallel to itself, then the given definition for "upwind leg" is consistent with both a path that is offset from the runway and a path that is in line with the runway. Unfortunately there is no single answer to that question, as the top answer explains; different schools of thought define parallelism to apply to a single line, or not.
However, there is a clear reason why you will hear a controller tell you to "EXTEND UPWIND" upon departing a runway. This phraseology, despite not appearing in the .65, is taught at the FAA Academy in OKC as correct phraseology and an acceptable technique. Indeed, the voice recognition software used in the tower simulators there will recognize "EXTEND UPWIND" as a valid instruction. So controllers who have been trained at the FAA Academy are sent out into the field armed with this tool in their toolchests and they are ready to use it.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Are aircraft wings airtight?
Is there any no air proof design for the wings? Or are  some of the parts are equivalent to outside pressure and some have gauge pressures?
Are some parts airtight and others not?

Yes, I ask about it and in detail if some parts are airtight or not of the wing structure?

I imagine it depends on the airplane. At least one is so far from being airtight that it's not even cat-tight.

If wings were airtight, as the plane climbs to altitude with lower air pressure, the wings would explode. Here’s an example of a chip bag in a commercial airplane with a cabin altitude equivalent to ~ 8,000' https://www.youtube.com/watch?time_continue=44&v=5QxOlatS9Fc

No part of my Cessna Cardinal wings are airtight. Even the fuel tanks are vented to allow air in to replace the fuel this is used.
The area around the flaps, flap motor aileron control cables, etc., none are airtight.

Structures on aircraft will typically not be air tight. Even pressurized fuselages with outflow valves closed will leak some amount of air that needs to be replaced. The pressurization puts a lot of strain into the skin and supporting structure. Fuel tanks, which have to be liquid tight, have vents to allow air in and out as the fuel level changes, and to allow overflow of fuel.
There would be no purpose to making parts like wings air tight, and as the aircraft climbs and descends the pressure differential would put a lot of force into the parts. The parts will be mostly sealed to prevent things like rain from getting in but will also have drains to allow any liquids to escape and for pressure to equalize.
One exception would be in flexible structures such as inflatable wings and parafoils where the pressure provides some rigidity.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Attempting to determine whether a flight took place
We had some houseguests recently who were scheduled to take American Airlines flight 2953 from SAV to ORD (May 16, 2019). They called and said that there had been a mechanical problem with the plane and that the flight had been cancelled. Since I had the flight information, however, I had already been tracking the flight status, and I saw that the plane was still scheduled to depart, and then I saw that it had taken off and landed, albeit with a delay. (The guests came on another flight the next day.)
I didn't confront the guests, of course, but I am curious to know whether there is any possible explanation for the discrepancy (i.e., other than that there was some other reason for the delay that they didn't wish to share with us). Could the flight have been cancelled and then rescheduled without their knowing it? Could the plane have been returned to ORD without passengers, and it showed up on the flight tracker as the regularly scheduled flight? Or is there some other possibility?

That flight did indeed happen:
https://flightaware.com/live/flight/AAL2953/history/20190516/1632Z/KSAV/KORD
It was scheduled for 12:22PM, but actually left the gate at 1:43PM (about 1 hour and 20 minutes late).
It was scheduled to land at 1:28PM, but actually landed at 3:25PM (about 2 hour late).
It looks like it took an extended detour to Missouri to avoid a weather system, resulting in the extra delay.
While the flight was not canceled, it is possible that passengers were offered free re-booking opportunities due to the weather system.  Your house guests may have taken that on the belief the flight might be canceled.

According to Flightradar24, the aircraft arrived 18 minutes late in SAV (12.10 ATA instead of 11.52 STA) on a flight from ORD. It departed SAV 1 hour 37 minutes late (13.59 ATD instead of 12.22 STD). Somehow, the turnaround took over an hour longer than scheduled. We can't be sure of the reason, but it may be due to technical reasons.
It is possible that the aircraft was flown empty back to ORD, but the rate of climb on departure was not exceptionally high compared to the same flight on other days. (although definitely not slow).


text:
Why don't all pilots receive at least basic instrument training?
Given the frequency with which non-instrument-rated pilots get themselves killed attempting to continue VFR flights into instrument conditions, why don't all pilots receive at least basic instrument training (at the very least, knowing how to interpret the navball and headingometer) during their initial flight training?

This depends on the country/regulator (e.g. the US does require it for private pilots), are you asking about a specific country?

@Pondlife: I was more thinking in general (but, if all U.S. pilots are required to be instrument-trained, then why are VFR-into-IMC occurrences still so lethal for said pilots?).

See this question. The only purpose of basic instrument training in the US is to be able to execute a 180° turn on instruments. The scenario is that you fly into a cloud at night, or into hazy conditions and suddenly realize you have no visible horizon. So you turn around and back into the clear conditions you just left. Piloting skills deteriorate quickly unless you lose them frequently, and by definition non-instrument pilots never practice them for real.

I learned (in the Netherlands) to read my instruments the very first time I took to the air with an instructor. I was also told to not rely on them too much but look out the window at all times for visual cues instead, and how to do that.

While the number of pilots who die flying VFR into IMC is tragic, it's a miniscule percentage of pilots overall. The vast majority of us take the risk seriously and either stay clear of bad weather or get the additional training to be able to handle it safely. The former have no need for the additional training, and the latter are already getting it.
Every pilot in theory knows how to fly by instruments. The problem is that it is surprisingly difficult to believe them when your body is telling you they're wrong. And once you spend a few hours training yourself out of that problem, you still have to spend dozens more hours learning how to fly in the IFR system, which is an entirely different beast than flying VFR--and to many (perhaps even most) pilots not of interest at all.
Also, flying is a perishable skill. Having flown IFR once years ago doesn't mean you can safely do it today, especially if it happens unexpectedly. Worse, having had training may give people a false sense of confidence in their skills, leading them to knowingly fly into bad weather that they would have avoided if they'd never had that training.
FWIW, using your logic, the FAA does now require a few hours of hood time to get a PPL, but the purpose of that is to demonstrate to students how they are not prepared to handle it, other than to make an emergency 180 and return to safety. It's like how students practice stalls so they know how to recover from (and ideally avoid) them, not to encourage pilots to stall their planes in normal flight. And the stats haven't changed.
In the end, the root problem isn't the lack of instrument training; it's poor judgment. Take away one way for high-risk pilots to kill themselves and they'll just find another, so nothing changes except the cost of training went up for the vast majority of pilots who didn't need saving in the first place.

"Worse, having had training may give people a false sense of confidence in their skills" so very, very true in so many areas of life.

I disagree that every pilot knows how to fly on instruments, even in theory. Cross-checking your instruments occasionally in VMC is a totally different thing from continuously scanning in IMC while also shooting an approach.

@Pondlife Hence "in theory".


text:
Why does the U.S. have so many general-aviation aircraft?
In a comment on my previous question about the paucity of South Korean general-aviation aircraft in the 1990s, @Pondlife pointed out (in response to my comparison between the numbers of GA aircraft per capita in South Korea and the United States) that, in their words:

Considering that just over 50% of the world's GA aircraft are in the US perhaps you should be asking why the US has so many, rather than why South Korea has so few? :-)

Why does the U.S. have such an extreme abundance of general-aviation aircraft (210,000 out of the total world population of 416,000 GA aircraft in 2016, according to the provided link), amounting to one GA aircraft for every 1,540 U.S. citizens (a per-capita abundance more than ten times the global average of one GA aircraft per 17,950 humans)?

Tangential point, but I suspect this is why you hear of more miracle landings in the U.S. than elsewhere in the world: airline pilots there are more likely to be experienced private aviators outside of work; while in many other countries, pilots are only as good as their companies can afford to train them, and structured trainings can only expose you to a limited number of conditions.

Actually on a per-capita basis, Canada is the most "heavily aviated" country in the world.
CANADA:
GA feet of 29000, total population 37 million.  One aircraft for every 1275 residents.
US
GA fleet of 211000, total population 327 million.  One aircraft for every 1550 residents.  Canada is roughly as wealthy, and has even more wide open spaces for its population, so it makes sense in that regard.

Combination of two factors

Wealth, particularly long term. The USA is a wealthy country that has been that way since the invention of flight. A large number of general aviation aircraft flying today are quite old 70's and 80's equipment makes up a large portion of the market and even 40's and 50's equipment can be found easily.
Area and population density. Europe and the Asian Pacific are densely populated areas. Cities are close to one another meaning commute times are short and airspace is highly regulated (those airports are close together as well). This makes it harder to fly and leaves less incentive as one is less likely to have a particular route in mind.

The only a few wealthy countries have large amounts of land, Australia, Canada, and the USA.

And Australia and Canada have far fewer people than the United States.

Exactly, together they have less than 1/5th the population of the USA.

Land of the free, that's why.

Part of the reason is also historical. After World War II was over there was a large number of veterans with flying experience returning to a country that suddenly had a massive surplus of small trainer aircraft.
While the US expended huge amounts of resources on World War II, it was largely untouched by the destruction that many other countries suffered. This meant that instead of having to rebuild, the US could focus on growing the economy. Though GA declined in the Great Depression and was suspended during World War II, there had been a similar situation after World War I that had gotten the industry started.
Small aircraft were much simpler then, with a much lower cost to build and certify. This made general aviation relatively much cheaper. Also, while the US certainly had a highway system, the interstate highway system had yet to be built. Commercial aviation service was not nearly at the scale it is today and was still fairly expensive. This made general aviation a much more reasonable option for traveling around a large country.

It is said that airplanes fly because of money and the Bernoulli effect, in that order. So the first cause is simply that there's a lot of wealth in the USA to support what is essentially a rich person's hobby. There are also a lot of airplanes still flying that were built during the boom years of aviation to support interest in the novelty of plane ownership (which is much less a novelty today), which tends to hold down the purchase price of a small plane. In addition, gasoline is much less expensive here than in western Europe so the operating costs will be lower.

Is the gasoline price differential actually a thing for avgas as well as for mogas?

@Sean If anything, it's worse for avgas than mogas. 100LL is getting expensive even in the US, and it's not available at all in many countries due to lead bans or insufficient demand.


text:
Why do aerobatic aircraft have separate tanks instead of a single acrotank?
I've been reading about fuel tanks in aerobatic planes such as the Extra 330, Edge 540, XA41 and I don't really understand why in the Extra 330 there are separated center tanks and an acrotank instead of a single bigger acrotank like in the Edge and XA41.
I guess it's something related with balance and center of the aircraft but the Extra has two center tanks and their relative positions between them and with the acrotank are small.

A long time ago aerobatic aircraft had just a single tank with a flop tube inside it.
This works "OK", but when you're doing something complicated with a half-empty or mostly-empty tank, it's too easy for the flop tube to ingest air instead of fuel.  This leads to performance problems with both the engine and the pilot.
The solution is to have a very small "acrotank" with a flop tube and an arrangement of fuel line plumbing such that the tank remains 90% full, 90% of the time.
In my Laser, there's a very large tube (2" to 3" diameter) that feeds from the bottom of the main tank to a tube at the top of the acro tank (with that tube extending inside the tank to near the bottom).  There's a small vent/return line.
When I'm right-side-up, the acro tank fills from the main tank in seconds.  When I'm inverted, the tube arrangement prevents most of the fuel from returning to the main tank.  Some returns via the vent/return line, but I still have about 5 minutes of sustained inverted flight fuel.


text:
Could a plane land on specialized vehicles if its landing gear failed?
This (fake) video has stirred up a lot of questions online about whether such a scenario would be possible in reality - and the answer to those questions appears to be a resounding "No, that's very implausible."
But most of those answers seem to focus on the specifics of the Nissan truck - it would exceed its rated weight tolerance, it probably wouldn't be able to match the plane's speed...
What if an airport had specialized vehicles for this sort of thing, Thunderbirds-style? Could such vehicles be built, or would the engineering challenges just be too great to overcome?

Closely related, maybe a dupe? Although this question asks about a specialized vehicle, not a general-purpose one.

Well, an aircraft carrier is a vehicle, isn't it?

Hmm... I was thinking the Semi powered by a jet engine might be just the vehicle for airliners. https://www.youtube.com/watch?v=qAkrzEqHHyA

Yes, it potentially could, its a common air show stunt, but would it, most likely not. There was a similar scenario where the gear failed in a similar way and the plane was simply landed delicately on the bad gear. There were lots of sparks but everyone was ok. Ultimately its not worth the risk of the truck driver (should the gear crush them) aircraft generally speaking can survive belly landings should something go wrong with the gear.

Smaller planes can pull it off, here is video from a weekend airshow somewhere.
http://digg.com/video/plane-lands-on-truck
Bigger planes, that's a lot weight. Even my Cessna Cardinal (4 passenger) could weigh close to 2500 lbs landing, basically a whole nother car on top of a truck.  Maybe a car carrier would be good, longer landing ramp, clear front to fly over if needed.
Jets, that's a lot more speed to match up with.

Another similar one: https://www.youtube.com/watch?v=ed-Bi4py6fk

I think for airliners it is perhaps borderline possible but highly impractical.
Lets assume a 737 and google around the internet for some rough figures.
A 737 apparently lands at 155 knots which translates to around 180 miles per hour and has a max landing weight of about 65 tonnes. Apparent 4-20% of that can be on the nose gear. So that is up to 13 tonnes on the nose gear and up to 31 tonnes on each main gear. Dynamic loads can potentially be quite a bit higher than the static weight, even more so if the landing gear is damaged.
So you are looking at a vehicle with the speed of a supercar and the weight handling capacity of a large truck. That is going to be a nontrivial engineering challenge.
Then you need some mechanism for the trucks to safely interface with the plane. This is further complicated by the fact that the gear could be in almost any state.
Plus you need a runway with enough room for the trucks to accelerate, rendezvous with the plane and then slow down safely. Probably at least double the length you need for a normal landing.
Plus the rendezvous itself isn't going to be an easy procedure. Sure stunt pilots can do similar things in small planes but those planes fly much slower and are much more manoeuvrable than an airliner.
And then finally you would have to convince the authorities that this hairbrained scheme is at least as safe as just landing the plane with broken gear.


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What is an “aeronautically qualified witness”?
Many older NTSB AARs, in their history-of-flight-and/or-accident-sequence sections, make mention of observations by an “aeronautically qualified witness” or witnesses, without any further explanation of what this term means.  Example (AAR 76-04, page 3 [paper]/6 [PDF]):

An aeronautically qualified witness, who was standing near the base of the radio tower when the plane struck, stated that the sky was overcast and that the bottoms of the clouds were obscuring the top 30 feet of the tower.  [...]

What, exactly, is meant by an “aeronautically qualified witness”?  Does it mean “a witness who is themself a pilot”?

An air traffic controller would also be "aeronautically qualified". Without a specific definition, I expect it means "one who is likely to know what they're talking about based on some relevant qualification".

It means a witness with credentials and knowledge that makes the witness competent to give a reasonably accurate description of an event, or an informed opinion, to an investigating authority or a jury.  Likely a pilot witness to a crash as in this case, who knows more about weather than the average person, or a mechanic witness in an investigation or lawsuit over maintenance.


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Why was the Akron’s spy basket installation so remarkably unstable?
A spy basket (also known as a cloud car) is a type of airship appurtenance consisting of a streamlined one-(usually)-person gondola attached to its host airship by a long cable; to use it, the host airship hides in or above a cloud layer, and the spy basket is then lowered, cum crewmember,1 from the airship down through the base of the cloud deck, and serves as an observation station in the (hopefully) clearer lower-altitude air, allowing the airship to know what happens to be below it without exposing its gigantic soft underbelly beneath the clouds.2
It was pioneered by the German armed forces during World War I, who used (or at least tried to use) it to drop bombs on Londoners3 without getting shot down; they eventually discarded the whole concept, but it was picked back up, five seconds over a decade later, by the United States Navy (the only post-1918 military user of rigid airships),4 when the U.S.S. Akron (ZRS-4) was built with provisions for the fitting and use of a spy basket.  The Akron’s spy basket was first deployed in April 1932.
It did not go well:

Soon after returning to Lakehurst to disembark her distinguished passengers, Akron took off again to conduct a test of the "spy basket"—something like a small airplane fuselage suspended beneath the airship that would enable an observer to serve as the ship's "eyes" below the clouds while the ship herself remained out of sight above them. The first time the basket was tried (with sandbags aboard instead of a man), it oscillated so violently that it put the whole ship in danger. The basket proved "frighteningly unstable", swooping from one side of the airship to the other before the startled gaze of Akron's officers and men and reaching as high as the ship's equator.[18] Though later improved by adding a ventral stabilizing fin, the spybasket was never used again.[26]

This was not, so far as I can tell, a problem faced by the German airship bomber crews of World War I (who were blessed with spy baskets that dangled placidly from their mounts), and I’m having trouble seeing why the Akron’s spy basket would exhibit such remarkably unstable behaviour, unless the first (and only) trial of her spy basket was held in severe turbulence (which, given the purpose and method of use of the spy basket, I’m having great trouble believing that her commanding officers would have allowed).  Can someone clarify this for me?

1: Presumably the one with the most favourable combination of good vision, non-acrophobia, and low rank.
2: It can also, if necessary, be used to determine if the cloud deck does, in fact, have a base, or if it extends all the way to ground level.  Obviously, due to the likelihood of eventual ground/water/tree/automobile/watership/Paul Bunyan contact in the event that the cloud layer is continuous to ground level, this is only advisable when the airship is in a stationary hover, unless you really don’t like the person manning the spy basket.
3: And probably some other people, too.
4: The German military, having lost World War I fair and square, was forbidden to build, use, possess, rent, touch, or think dreamily about any sort of aircraft at all, lighter-than-air or heavier-than-air, while the British military, in order to save money, scrapped its fleet of rigid airships at the end of the war and cancelled all those not yet delivered (with the sole exceptions of one they were going to sell to the Americans but which, due to poor judgement on both the designers’ and the pilot’s parts, crashed while they were showing it off for its prospective customers and one which was hastily reconfigured for passenger service but proved too small for the role).

Very interesting bit of physics going on here with the cable car swinging "up to the equator" of the airship.  Perhaps something similar to what happened with the "Galloping Gertie" Tacoma Narrows suspension bridge in 1940.
The suspension cable could be hundreds of feet long.  In a cross wind, if the mass to surface area (wind pressure) of the airship and the cable car did not match, the cable car would swing to the side and then back.  If the wind and the swing motion became harmonicly re-inforceing, each swing becomes a little higher, eventually reaching a full 180 degree sweep.
The elasticity of a cable several hundred feet long may have contributed, stretching a little at the bottom of the arc and contracting a little at the top of the swing, helping propell the object back in the other direction.
Another factor have been a very slight difference in airspeed as the car swung away from the wind, pulling the airship with it, as compared with the return swing back into the wind.  Each swing adds more energy to the system.
Without "dampers", the cable and car are wildly out of control.  The fix, amazingly, may have been to fashion adjustable "sails" for the cable car. Some wind tunnel testing may have been
helpful as well.

But what's driving the resonance? For it to be the wind, the wind would have to be gusting with about the same frequency as the pendulum, which seems unlikely.

What drives the resonance is some form of energy storage like a spring.  These cables were up to 1/2 mile long, enough to stretch a little.  Very surprised they did not study this and try again.  I would trace out the exact arcs and see if the movement down wind of the airship added energy as well.  A piloted version could have used speed brakes on the return swing to slow it.

@DavidRicherby Mode coupling in a free hanging pendulum means that a perturbation in any two non orthogonal directions will begin to precess. If one axis is damped (say the one in direction of wind/airflow) then the motion will "accumulate " in the undamped mode. The spring of the pendulum will exacerbate this coupling but is not required for this to happen , only a fee hanging pendulum with minimal damping. As robert points out a damping moment in the form of speed brakes would solve the issue. Intuitevly this behavior can be seen in a foucault pendulum.

@crasic thank you so much. I was seeing the relative motion of the pendulum and the airship as slightly different when the pendulum swung back upwind, and the airship continued downwind.  I believe the energy is added when the pendulum reaches the top of the upwind arc and gets "pulled" by the airship back into its downwind arc.  This would not happen if the top of the pendulum was fixed (unless the fixture started to sway in the pendulum's harmonic frequency).


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Why was the B-52's wet wing more susceptible to fatigue than its dry wing, and not less?
After years of carrying all its fuel in the fuselage, the B-52 transitioned to a wet wing (where the wing structure is used as one or more fuel tanks) with the B-52G.
This caused severe problems with metal fatigue in the wing structure (the wet wing increased structural stresses by approximately 60% compared to the A-F's dry wing, according to Wikipedia), which was already an issue due to the increased aerodynamic stresses associated with a recent switch from a high-altitude to a low-altitude nuclear mission profile (the high-altitude mission profile having become unsurvivable due to advances in Soviet SAM technology).
However, storing fuel in the wings of an aircraft normally decreases the stresses on the wing structure (by putting as much of the aircraft's weight as possible in the part of the aircraft that generates the lion's share of its lift), alleviating fatigue and increasing the aircraft's fatigue life.  What made the B-52 different in this regard?

You are making two false assumptions regarding the B-52 change to a wet wing. 1) Early B-52s did not carry ALL their fuel in the fuselage. 2) It was not the wet wing design that resulted in fatigue cracks. It was the change to a lighter aluminum alloy wing design that caused problems.

In general, flight loads bend a wing upwards and ground loads bend a wing downward. When you add fuel to the wing it resists the upward loads in flight better, but increases the downward bending on the ground a lot more. Fatigue damage is cause by the cycling of load, so a wing skin that is in heavy compression on the ground and moderate tension in flight can have more fatigue than one that is in light compression on the ground and a heavy tension in flight. @MikeSowsun's answer is create with regard to the B-52, though.

Earlier B-52s did not carry all their fuel in the fuselage. They also carried their fuel in the wings using rubber bladder-type wing tanks.
The new B52G “wet wing” was a weight savings measure which also increased the fuel capacity over the bladder-type in-wing tanks. The metal fatigue of the new wing was due to a poor structural design which was supposed to save weight.
http://www.joebaugher.com/usaf_bombers/b52_15.html

The B-52G had a lighter structure than previous versions in order to save weight, but it carried more fuel. This meant that fatigue problems resulting from the structural flexing generated by the stresses of low-level flying and midair refuelling manifested themselves earlier in the life cycle of the airframe. This was especially the case in the region of the wing structure where most of the weight savings had been achieved by using an aluminum alloy. Fatigue cracks got so bad that stringent flying restriction had to be imposed, pending modifications. In May of 1961, a program was approved in which the wings were modified and strengthened as part of the regular IRAN schedules for the aircraft in the B-52 fleet. The project was finally completed in September of 1964.

Early B-52B,C,D fuel chart showing multiple wing tanks.

The decrease in fatigue is not a property that's automatically conferred to the plane by the introduction of wet wing - it's a product of smart design. Usually the decision to use a wet wing is made during the initial design of a plane, and the support structure of the wing is built accordingly, so that it can take the dynamic strains that occur upon refueling (in case of military aircraft - possibly in-air refueling) and upon spending fuel from the tanks. The initial design of the (initially dry-wing) B-52, obviously, did not account for putting such loads on wing structure (the additional internal wing tanks carried 17 tons of fuel!).
During the modification to wet wing, the wing should've been reinforced to counteract the additional load. Instead, the range of the bomber was prioritized: G and H models were lighter than previous models, mostly due to extensive usage of aluminium and titanium parts, particularly - in wing structure; at the same time they carried much more fuel. Alloys used were less fatigue-resistant than ones used in previous models. The testing done in 1960 by Boeing and Air Force confirmed that combined with additional stresses induced by change of usage (i.e. low altitude missions and wing tanks operation) this significantly shortened the operational life of the aircraft. This result was made painfully clear by B-52G that crashed near Gainsborough in January 1961. That led to a new modification program being endorsed by the Air Staff in May 1961, which was finished in 1964 - it involved reinforcing the wings and other areas subject to increased fatigue ("The wing structural improvement program, carried out as ECP 1050, replaced the wing box beam with a modified wing box that used thicker aluminum. It also installed stronger steel taper lock fasteners in lieu of the existing titanium fasteners; it added brackets and clamps to the wing skins, added wing panel stiffeners, and made at least a dozen other changes. Finally, a new protective coating was applied to the interior structure of the wing integral fuel tanks." - Encyclopedia of US Air Force Aircraft and Missile Systems: Post-World War II bombers, 1945-1973, Volume 2).

But the question is, why is there additional load? If the fuel is in the fuselage, the weight has to be transferred to the wings through the main spar, creating a bending moment on it. When the fuel is moved into the wings, it is at the place where lift is created, so less force to transfer and less bending moment. So… there must be something else…

@JanHudec I modified the answer with more info on the wing structure modification. Aside from that - the weight of fuel still affects the internal wing structure, even if the bending moment is being compensated by lift forces; the wing is not a single-piece solid body, and forces acting on it from inside and from outside induce stress on internal structure even if they are largely compensate each other.

Also, stress is not caused by load itself, but by load cycles, isn't it? So it they made it a bit lighter as the magnitude of stress was lower, they could have easily made it more susceptible to fatigue by mistake.

The B-52G wing was not a modded "dry" wing. It was a totally new wing design that along with other changes, went from bladder fuel tanks to wet wing tanks.


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Why do oblique-wing aircraft suffer from significant aerodynamic cross-coupling?
Oblique-winged aircraft (which have one long wing that pivots around a central point, so that the wing is straight at low speeds but strongly-swept - one side forwards, the other side back - at high speeds), while theoretically more efficient than conventional aircraft, have a number of issues which have prevented them from seeing any significant use; one of these problems are their various unusual cross-coupling modes, including a significant tendency towards pitch-roll coupling (where pitching up or down makes the aircraft roll to one side or the other, and vice versa) and a need to hold a significant bank angle just to maintain coordinated flight and keep from slipping sideways (even with no thrust asymmetry), all of which become increasingly severe at high sweep angles.
For example (emphasis added):

McMurtry found the handling qualities were very close to the simulator, but that the aircraft’s asymmetry resulted in unusual trim requirements, asymmetric stall and inertial coupling. [...]  For instance, the AD-1 required about 10° of bank in order to trim the aircraft with no sideslip at 60° wing sweep.
[...]
As hoped, the low-cost, low-speed, low-technology AD-1 successfully demonstrated the concept of a manned oblique wing aircraft, sweeping the wing to a maximum of 60°.  The aircraft experienced cross coupling between pitching moment and aileron deflection.  This pitch-roll coupling and the aeroelastic effects contributed to unpleasant handling qualities at sweep angles above 45°.  [...]  [A Summary Of A Half-Century of Oblique Wing Research, pages 20-21.]
The proposed wing pivot was also to be canted so that at 0° sweep, the wing was canted to 0°, but at 65° sweep it would be canted to 10°.  The wing incidence would also have been increased (a standard feature of the F-8) with higher sweep angles.  These features were expected to reduce aerodynamic cross-coupling, such as the necessity to bank the whole aircraft to trim sideslip, as found in the AD-1 flight testing.  [...]  _[A Summary Of A Half-Century of Oblique Wing Research, page 23.]_
Flight tests of AV1 and AV2 proved the design to be controllable in both pitch and roll.  Noble states, “The elevon mixing is standard symmetric and the aircraft responds equally to left and right inputs.  There is some slight roll coupling associated with up and down elevator.  I believe this can be eliminated with fine tuning of the elevon mixing.”  _[A Summary Of A Half-Century of Oblique Wing Research, pages 32-33.]_

Why do oblique wings come with these strange cross-couplings?

Why would they not?

With the wing swept forward, a positive lift force on that wing will bend the wing upward and locally increase the angle of attack, especially to the outboard of a wing. The same thing happens on the wing swept backwards, only an upward deflection from lift force will reduce the local angle of attack, likewise to the greatest extent toward the wing tip where it is deflected upward the most. One wing will end up generating more lift than the other, which will require a rolling input from the aileron to cancel out. One could attempt to balance this out with different twist profiles of each wing, but that would only solve the issue in one g flight.
When, for example, the elevator is deflected to induce a positive pitching moment, the angle of attack on the wing increases and lift force increases. The increased lift generates an increased deflection of the wing. However, as outlined above, the impact on the local angle of attack due to that increased deflection is opposite between the left and right wings. This produces a coupled roll effect.
Finally, the asymmetric lift and therefore induced drag, in addition to the use the ailerons to counter this rolling tendency, will produce adverse yaw. To eliminate the sideslip you would need to use substantial rudder input, or a bank in the opposite direction of the adverse yaw induced sideslip.
Edit:
For a really simple visualization of the wing deflection inducing a local angle of attack change, take a piece of paper and hold it flat in the air in front of you, with the long edge facing you but turned away by 45 degrees (imagine you are looking at the leading edge of a 45 degree swept wing).  Bend the short end of the paper sheet that is farthest from you (the wing 'tip') upward. Your perspective is aligned with the freestream aerodynamic flow, and by bending the tip upwards you can now see the top of the outboard wing. For a normal rear swept wing this produces washout and unloads the outboard wing. Now imagine the short end of the paper sheet that is closest to you is the wing tip. Hold the wing 'root' fixed and bend the new wing tip up. The upward deflection increases the local angle of attack at the wing tip, rather than decreasing it as before for the rear swept wing.
As an aside, this is also the reason why forward swept wings are often aeroelastically unstable. An increase in the lift force produces more lift force due to the deflection, a positive feedback that can much more rapidly lead to divergent flutter!